How turbochargers work
by Chris Woodford. Last updated: October 11, 2016.
There's no such thing as a perfect invention: we can always make something better, cheaper, more efficient, or more environmentally friendly. Take the internal combustion engine. You might think it's remarkable that a machine powered by liquid can hurl you down the highway or speed you through the sky many times faster than you could otherwise travel. But it's always possible to build an engine that will go faster, further, or use less fuel. One way to improve an engine is to use a turbocharger—a pair of fans that harness waste exhaust power from the back of an engine to cram more air into the front, delivering more "oomph" than you'd otherwise get. We've all heard of turbos, but how exactly do they work? Let's take a closer look!
Photo: A typical automobile turbocharger uses a pair of snail-shaped fans, like this. The one you see here is a Garrett GT2871R, just about to be fitted to the engine of a Pontiac G8. Photo by Ryan C. Delcore courtesy of US Navy.
What is a turbocharger?
Have you ever watched cars buzzing past you with sooty fumes streaming from their tailpipe? It's obvious exhaust fumes cause air pollution, but it's much less apparent that they're wasting energy at the same time. The exhaust is a mixture of hot gases pumping out at speed and all the energy it contains—the heat and the motion (kinetic energy)—is disappearing uselessly into the atmosphere. Wouldn't it be neat if the engine could harness that waste power somehow to make the car go faster? That's exactly what a turbocharger does.
Car engines make power by burning fuel in sturdy metal cans called cylinders. Air enters each cylinder, mixes with fuel, and burns to make a small explosion that drives a piston out, turning the shafts and gears that spin the car's wheels. When the piston pushes back in, it pumps the waste air and fuel mixture out of the cylinder as exhaust. The amount of power a car can produce is directly related to how fast it burns fuel. The more cylinders you have and the bigger they are, the more fuel the car can burn each second and (theoretically at least) the faster it can go.
One way to make a car go faster is to add more cylinders. That's why super-fast sports cars typically have eight and twelve cylinders instead of the four or six cylinders in a conventional family car. Another option is to use a turbocharger, which forces more air into the cylinders each second so they can burn fuel at a faster rate. A turbocharger is a simple, relatively cheap, extra bit of kit that can get more power from the same engine!Photo: Two views of an oil-free turbocharger developed by NASA. Photo courtesy of NASA Glenn Research Center (NASA-GRC).
How does a turbocharger work?
If you know how a jet engine works, you're halfway to understanding a car's turbocharger. A jet engine sucks in cold air at the front, squeezes it into a chamber where it burns with fuel, and then blasts hot air out of the back. As the hot air leaves, it roars past a turbine (a bit like a very compact metal windmill) that drives the compressor (air pump) at the front of the engine. This is the bit that pushes the air into the engine to make the fuel burn properly. The turbocharger on a car applies a very similar principle to a piston engine. It uses the exhaust gas to drive a turbine. This spins an air compressor that pushes extra air (and oxygen) into the cylinders, allowing them to burn more fuel each second. That's why a turbocharged car can produce more power (which is another way of saying "more energy per second"). A supercharger (or "mechanically driven supercharger" to give it its full name) is very similar to a turbocharger, but instead of being driven by exhaust gases using a turbine, it's powered from the car's spinning crankshaft. That's usually a disadvantage: where a turbocharger is powered by waste energy in the exhaust, a supercharger actually steals energy from the car's own power source (the crankshaft), which is generally unhelpful.
How does turbocharging work in practice? A turbocharger is effectively two little air fans (also called impellers or gas pumps) sitting on the same metal shaft so that both spin around together. One of these fans, called the turbine, sits in the exhaust stream from the cylinders. As the cylinders blow hot gas past the fan blades, they rotate and the shaft they're connected to (technically called the center hub rotating assembly or CHRA) rotates as well. The second fan is called the compressor and, since it's sitting on the same shaft as the turbine, it spins too. It's mounted inside the car's air intake so, as it spins, it draws air into the car and forces it into the cylinders.
Photo: The essence of a turbocharger: two gas fans (a turbine and a compressor) mounted on a single shaft. When one turns, the other turns too. Photo courtesy of NASA Glenn Research Center (NASA-GRC).
Now there's a slight problem here. If you compress a gas, you make it hotter (that's why a bicycle pump warms up when you start inflating your tires). Hotter air is less dense (that's why warm air rises over radiators) and less effective at helping fuel to burn, so it would be much better if the air coming from the compressor were cooled before it entered the cylinders. To cool it down, the output from the compressor passes over a heat exchanger that removes the extra heat and channels it elsewhere.